In the semiconductor industry, in general substrates of different sizes, shapes and materials are joined to one another. The joining method is called wafer bonding. Wafer bonding is roughly divided into permanent and temporary bonding. In permanent bonding a no longer detachable connection between the substrates is formed. This permanent connection takes place for example by interdiffusion of metals, by cation-anion transport in anodic bonding or by the formation of covalent bonds between oxides and/or semiconductor materials in fusion bonding, and in the crosslinking of polymers in bonds with organic cements.
In temporary bonding mainly so-called bonding adhesives are used. They are adhesives which are applied to the surface of one or both substrates by a coating method in order to act as adhesive between the substrates.
In all bonding methods bonders are used to apply a blanket pressure as uniform as possible to the substrates to be bonded to one another. Here it is of very great importance that the pressure distribution along the surface of the substrates is optimum. Otherwise voids can arise due to gas inclusions, by squeeze-out, by absent grain growth and by nonuniform layer thicknesses.
Pressure inhomogeneities can be attributed mainly to poorly manufactured pressure disks, wafer chucks or to their wear. Furthermore the different elastic properties of the components which generate the compressive load on the substrates are responsible for many pressure inhomogeneities. This is mainly the case when the moduli of elasticity of the components of the bonder are smaller than the moduli of elasticity of the substrates to be bonded or the layers which are located between the substrates. In order to obtain a corresponding elasticity of the bonder, pressure disks, wafer chucks or compensation disks produced from special materials such as graphite are used to obtain the optimum bonder configuration. In very many cases for example graphite compensation disks of corresponding size are fixed between the pressure disk and the piston. The use of these compensation disks depends on the respective configuration and the bond type. Graphite compensation disks have been used most often to date since they deform very well, are temperature-stable and have a corresponding modulus of elasticity under full compressive loading. Generally the use of these compensation disks improves the pressure homogeneity.
Therefore methods are known for obtaining quantitative information about the pressure distributions along the pressure surfaces.
The most frequently used method is the evaluation of the color information on paper which is colored to different degrees by bursting of color balls at high pressure. Although this method is current practice, it has disadvantages. Cutting out the films, installing them between the pressure bodies and removing them are accordingly time-consuming. Moreover the material cannot be used under thermal loading due to high temperature sensitivity; this also causes problems in the reproducibility of the results. Another problem is the numerical evaluation of the pressure data, which evaluation is not reliable or reproducible.
A second already known method was disclosed in WO2012/167814A1. This method is based on the evaluation of the deformation of fluid particles caused by the compressive load. This method does not allow in-situ measurement of the pressure distribution between the pressure bodies either.
A third, already known method uses a measuring device of hundreds of locally distributed pressure sensors which must be produced on a corresponding substrate, which method is complex and especially costly. Production takes place using microsystems technology. The microsensors are MEMS and/or semiconductor elements which can control a current, depending on the respective compressive loading.